1. Field of the Invention
This invention concerns a capacitive measuring device. More specifically the invention relates to a capacitive measuring device comprising a scale provided with an array of scale electrodes and a transducer provided with an array of transducer electrodes, the two electrode arrays being coupled capacitively in such a way that electrical signals can be produced which depend upon the resulting coupling and thus the relative position of the transducer and the scale.
2. Description of the Related Art
Devices of this type are used, for example, in instruments for measuring length or angles such as sliding calipers, or measuring columns or gauges for angles. They have the advantage of precise electronic measurement and are more economical than optical measuring systems. The patents or patent applications JP-A-12455283; U.S. Pat. No. 4,878,013; EP 0 248 165; U.S. Pat. No. 5,225,830; JP 1841668; JP 1783036; U.S. Pat. No. 5,239,307; EP 0 413 922; EP 0 404 980 and EP 0 400 626, among others, describe examples of known devices.
Generally speaking, these devices comprise a scale formed by a printed circuit or a glass substrate on which the scale electrodes are disposed. The transducer is composed of a second printed circuit equipped with an array of transducer electrodes placed facing the scale electrodes. The scale electrodes and the transducer electrodes thus constitute two arrays of capacitors, mobile with respect to one another. The capacitive coupling between the transducer electrodes and the scale electrodes varies according to the relative position of the transducer with respect to the scale. This information is evaluated in order to display the position measurement.
In these devices, the precision of measurement attainable is limited by different factors such as geometrical imperfections of the transducer or of the scale, in particular inaccuracies in mutual placement, surface unevenness or irregularities, or by imprecise geometry of the transducer or scale electrodes. In particular, the transducer electrodes are generally made using known techniques for manufacturing printed circuits. It is difficult with these techniques to obtain a precision in the geometry of the electrodes which is compatible with the desired smallest measurable quantities. The scale suffers from similar problems, at least when it is produced from a printed circuit. Moroever mechanical errors, due, for example, to imprecise construction or to dilatation cause changes in the spacing between the transducer and scale electrodes, and thus variations of the measured capacitances difficult to foresee or correct. In the aforementioned patents, several ways of placing electrodes on the transducer and the scale have been envisaged which permit at least partial compensation of the different factors leading to errors and which increase the precision.
In the Swiss patent CH 648 929 (TESA), the transducer electrodes are divided into four groups (A1, A2, B1, B2), phase-shifted with respect to one another by 90.degree.. Each group comprises three distinct electrodes, which allows certain errors to be distributed and averaged out at least partially.
However, in the aforementioned patent, all the electrodes in each electrode group are placed in the same corner of the transducer. In FIG. 1 of the patent, it is to be noted that all the electrodes of group A1 are placed in the upper left corner of the transducer, all those of group B2 in the lower left corner, and so on. This device is consequently very sensitive to errors in the positioning of the transducer opposite the scale and, in particular, in the pivoting of the transducer about its longitudinal axis.
In the European patent EP 0 404 980, for example, the transducer electrodes are equally distributed in four groups (1, 2, 3, 4) phase-shifted with respect to each other by 90.degree.. In FIG. 4 of that patent, each group is composed of 5 distinct electrodes distributed more or less over the whole surface of the transducer in such a way that the groups are intermixed. The successive electrodes thus belong respectively to groups 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3 and so on. An identical succession of electrodes thus repeats itself periodically.
The U.S. Pat. No. 4,878,013 (Andermo) also proposes various ways of disposing the electrodes on the transducer. In particular, FIGS. 3 to 5 of that patent illustrate different ways of distributing the electrodes on the transducer so as to distribute the electrodes of each group (1 to 6) more or less uniformly over the whole surface of the transducer. The different groups of electrodes are thus equally intermixed. On the transducer of FIG. 4 of this patent, the successive electrodes belong respectively to groups 1, 3, 5, 4, 6, 2, then the succession repeats itself periodically. In the case of a transducer having a large number of electrodes, for example more than 50, distributed into a fairly small number of groups, for example 6, the same succession of electrodes repeats itself several times. With the embodiments of FIGS. 2 and 4, the succession repeats itself certainly less often, but the frequency of patterns thus formed remains high.
Owing to the techniques used in particular to manufacture the scale, and especially the transducer, certain geometrical inaccuracies repeat themselves periodically. This is the case, for example, for certain errors in the positioning or surface variations of the electrodes caused by the printing technique used. When the frequency of repetition of these errors is in a whole number ratio to the frequency of repetition of patterns of electrodes on the transducer, the resulting errors on all the electrodes of a group can accumulate instead of be averaged out. The correct functioning of the device is thus not ensured.
Among other things, since the succession of electrodes on the transducer repeats itself, the electrodes of each group are found regularly placed next to electrodes of another same group. For example, in FIG. 4 of the aforementioned patent, the electrodes of group 3 are always found between those of group 1 and those of group 5. The crosstalk between the electrodes of group 3 and those of group 1 is thus much greater than the coupling between the electrodes of group 3 and those of group 6, for example. This results in functioning asymmetries which can give a false measurement reading.
The European published application with the publication number EP 0 537 800 (Mitutoyo) and the European patent EP 0 400 626 (Mitutoyo) describe a transducer in which the electrodes have a variable shape and surface. This configuration permits compensation of certain errors in parallelism and pivoting of the transducer. The specific problems connected with the periodicity of electrodes remain however. Moreover, the capacitive coupling obtained is limited by the size of the electrodes, certain ones being very small. Therefore, to obtain a coupling sufficient for measurement, it is necessary for the transducer to move at a very slight distance above the scale, which poses problems of production mechanics.